At the wheel of his resurgent red car, the German won the Malaysian grand prix while Ferrari teammate Kimi Raikkonen came close to challenging for victory two weeks ago in Bahrain.

"We are trying to bother Mercedes," Vettel told Welt am Sonntag newspaper, "and we must be right behind them and pounce if something goes wrong with them. But it is also important that we are not passed by other teams.

"Mercedes still have the strongest package," he acknowledged. "You don't make up such a deficit quickly.

"Ferrari have taken a step forward in the winter but it will take a while to be on the same level as Mercedes," said Vettel.

However, Finland's MTV3 reports that Raikkonen may benefit from a Ferrari engine upgrade this weekend in Barcelona.

And the Maranello team will also reportedly race a raft of aerodynamic upgrades at the Spanish grand prix.

"Yes, I expect that Ferrari will be even closer to Mercedes" there, said MTV's F1 expert Ossi Oikarinen.

But F1 legend Gerhard Berger said he is not sure Vettel is now close to being able to mount a serious challenge for the 2015 title.

"Ferrari did a giant step over the winter," he told Auto Bild. "Whether it is enough for the title, I doubt it. For me, Mercedes' lead still seems too big."

Vettel's former boss and mentor at Red Bull, however, Austrian Dr Helmut Marko, is more confident.

"Sebastian is already going for this year's world title," he said, referring to the fact that Vettel is just a point behind championship leader Lewis Hamilton's Mercedes teammate, Nico Rosberg, after four races.

"Vettel has shown all his critics who told him he had forgotten how to drive a car last season," Marko added.